This invention relates, in general, to apparatus for heat fusing toner images onto a copy paper in a xerographic reproducing apparatus and, more particularly, to stripping apparatus suitable for stripping copies from a fuser roll or similar fuser structure.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual image can be either fixed directly upon the photosensitive member or transferred from the member to a sheet of plain paper with subsequent affixing of the image thereto.
In order to permanently affix or fuse an electroscopic toner material onto a support member by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to be absorbed to some extent into the fibers of the support member which in many instances constitutes plain paper. Thereafter, as the toner material is cooled, solidification of the toner material occurs causing the toner material to be firmly bonded to the support member.
In both the electrographic as well as the xerographic recording arts, the use of thermal energy for fusing toner images onto a support member is old and well-known.
One approach to thermal fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members at least one of which is either externally or internally heated.
During operation of a fusing system of the above-described type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll to thereby produce heating of the toner images within the nip. By controlling the heat transferred to the toner and by the provision of proper roll surface materials virtually no offsetting of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the hot offset temperature of the toner whereat the toner particles in the image areas of the toner would liquify and cause a shearing action in the molten toner to thereby result in offset. Shearing occurs when the inter-particle or cohesive forces holding the viscous toner mass together is less than the surface energy or adhesive forces tending to offset it to a contacting surface such as the fuser roll.
Occasionally, however, extraneous toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof; by imperfections in the properties of the entire surface of the roll, or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, extraneous toner particles may be transferred to the surface of the fuser roll beyond the nip, with subsequent transfer to the back-up roll during periods of time when no copy paper is in the nip and before the back-up roll can be moved out of contact with the fuser roll.
It will be appreciated that in order to prevent such toner particles being transferred to the copy paper it is necessary to remove the toner particles from the fuser roll and/or the back-up roll. It will be further appreciated that if enough toner accumulates on the back-up roll the paper feed will be affected.
One arrangement for minimizing the foregoing phenomena, commonly referred to as "offsetting," has been to provide a fuser roll with an outer covering or sleeve of polytetrafluoroethlene, commonly known as Teflon, to which a release agent such as silicone oil is applied. Silicone based oils, which possess a relatively low surface energy, have been found to be a material that is suitable for use in the heated roll fuser environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to thereby form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from adhering to the fuser roll surface. Although the low surface energy oils generally act as a non-wetting fluid in regard to most support materials, it has been found that a mechanical flowing of the release agent from the roll onto the support material will occur if an excess of oil is allowed to accumulate in a region where it can come into contact with the copy paper. Accordingly, the amount of oil applied to the roll surface is generally metered under controlled conditions to maintain a relatively thin coating of the release agent on the roll surface.
As the copy sheet passes out of the nip formed by the fuser roll and the back-up roll, it is necessary to provide means such as stripper blades or finger-like members to insure that the copy paper proceeds along a predetermined path from the nip of the fuser roll and back-up roll to a conveyor belt or the like for moving the copy paper towards the exit of the machine or apparatus. Such stripper members usually contact the surface of the roll and it has been found that due to such contact the oil forming the release agent tends to accumulate at the point of contact near the leading edge of the stripper member. It has been found that such accumulation will cause excess amounts of oil to be picked up by the copy paper thus adversely affecting the quality of the copy produced.
Furthermore, excessive contact pressure between the stripper members and the fuser roll can produce excessive wear of one or both of the foregoing. Excessive forces are attributable to the weight of the copy paper when it contacts the stripper members.
It is therefore the primary object of the present invention to provide an improved roll fusing device for fixing toner images.
Another object of the present invention is to provide improved means for removing a copy sheet from the surface of a heated fusing roll.
Yet another object of the present invention is to provide a stripping device in conjunction with a heated fusing roll which minimizes the contact forces between stripper members and a fuser roll when the stripper members are contacted by the copy paper.